1. Field of the Invention
The present invention relates to an image display device that modulates light by using liquid crystal modulation elements (liquid crystal display panels or liquid crystal display elements) and that displays images by using modulated light, and more particularly, to a projector display device that projects modulated light.
2. Description of the Related Art
Liquid crystal projectors using liquid crystal modulation elements, which serve as two-dimensional pixel optical switches, as image modulation units of projection display devices, are known. As the liquid crystal modulation elements used for liquid crystal projectors, twisted nematic (TN) liquid crystal elements and vertical arrangement nematic (VAN) liquid crystal elements are mainly used.
The liquid crystal modulation elements provide retardation to light waves passing through a liquid crystal layer to change the polarization condition of the light waves by using the electrically controlled birefringence (ECB) effect, thereby forming image light.
In the liquid crystal modulation elements that modulate the intensity of light by using the ECB effect, since a voltage (electric field or potential difference) is applied to the liquid crystal layer, ionic materials in the liquid crystal layer are migrated. If the application of a direct current (DC) to the liquid crystal layer continues, the ionic materials are attracted to either of the two electrodes opposing each other with the liquid crystal layer therebetween. Then, part of the voltage applied to the liquid crystal layer is canceled by the voltage formed by the ionic materials, which makes it difficult to apply a voltage having a desired intensity level to the liquid crystal layer.
To solve this problem, the following two methods are generally employed. One method is a line inversion drive method in which a voltage is switched at 60 Hz by inverting the polarity of the voltage for every other pixel line, i.e., line by line. The other method is a field inversion drive method in which a voltage is switched at 60 Hz by inverting the polarity of the voltage for every other pixel field, i.e., field by field. By preventing the voltage applied to the liquid crystal layer from being biased to one polarity, the uneven distribution of ionic materials (i.e., the generation of a voltage formed by the ionic materials in the liquid crystal layer) can be prevented.
However, the migration of the ionic materials is not the only reason for the fluctuations of the practical voltage applied to the liquid crystal layer (such a voltage is hereinafter referred to as the “effective voltage”). For example, in a nonconductive film of an insulator (such as a liquid crystal alignment film, a reflection-enhancing film, an inorganic passivation film for preventing metal elution, etc.), electric charge, such as electrons or holes, itself is sometimes trapped. This enhances charging on the interface of the nonconductive film, and this electrostatic charging may change the effective voltage of the liquid crystal layer. With this electrostatic charging, if the above-described liquid crystal modulation elements are driven by one of the above-described inversion drive methods, the difference between the absolute value of a positive potential difference (voltage) and the absolute value of a negative potential difference (voltage) becomes large, causing the occurrence of flicker. That is, the brightness when a positive potential difference is applied and the brightness when a negative potential difference is applied become different from each other, thereby giving rise to a phenomenon where a bright image and a dark image are alternately displayed at a frequency of 60 Hz, i.e., flicker occurs. This phenomenon (flicker) can be observed by the human eye if the difference between the absolute value of the positive potential difference and the absolute value of the negative potential difference becomes 200 mV or higher.
Flicker due to the electrostatic charging occurs when the two electrodes with the liquid crystal layer therebetween are made of the same material (mainly, in transmissive liquid crystal modulation elements), and it is even noticeable when the two electrodes are made of different materials (mainly, in reflective liquid crystal modulation elements).
A solution to the problem of flicker due to electrostatic charging is disclosed in, for example, U.S. Pat. No. 7,038,748. In this publication, a work function adjusting film layer is formed on a reflection pixel electrode, and the work function (Fermi level) of the reflection pixel electrode is set to be ±2% in relation to the work function (Fermi level) of a transparent electrode (indium tin oxide (ITO) film electrode) opposing the reflection pixel electrode. With this configuration, the charging on the interface of the liquid crystal layer can be suppressed, which would otherwise cause flicker or sticking.
More specifically, to allow electric charge to be trapped, excitation hopping of the energy potential of an insulating film between the liquid crystal layer and an electrode is necessary. In the technique disclosed in U.S. Pat. No. 7,038,748, the probability of the occurrence of excitation hopping from the mirror electrode and that from the ITO electrode are set to be close to each other so that the same amount of electric charge is trapped on either side of the liquid crystal layer. With this arrangement, although the voltage applied to the liquid crystal layer by the field inversion drive method shifts as a potential, the magnitude of the voltage remains the same. Accordingly, the voltage applied to the liquid crystal layer is reactively operated by the relative value between the opposing electrodes due to the ECB effect, and the operation of the liquid crystal is not changed.
According to the technique proposed in U.S. Pat. No. 7,038,748, the probability of the occurrence of excitation hopping from the mirror electrode and that from the ITO electrode becomes close, but it is difficult to set the amounts of charging due to excitation hopping to be completely the same. Accordingly, the charging on the interface of the liquid crystal layer gradually increases in accordance with the operating time of the liquid crystal modulation elements. In particular, in terms of the long-term reliability of the liquid crystal modulation elements, i.e., as the driving time of the liquid crystal modulation elements becomes longer, the potential difference between the mirror electrode and the ITO transparent electrode opposing each other reaches several hundreds of millimeter voltages. This phenomenon can be more easily observed as the photon energy input into the liquid crystal modulation elements is higher and as the light-quantity total energy is higher.
Additionally, if the potential difference between the mirror electrode and the ITO transparent electrode is generated due to the charging on the interface of the liquid crystal layer, the following problem occurs. If the application of a constant DC voltage to the liquid crystal layer continues, a minute amount of ionic materials in the liquid crystal layer is attracted to one of or both the interfaces of the liquid crystal layer close to the opposing electrodes. Then, the ions adhering to the interfaces of the opposing electrodes are moved in accordance with the magnitude of the amplitude potential of the field inversion driving, and thus, the amounts of ions adhering to the interfaces of the opposing electrodes become different depending on the magnitude of the amplitude potential. That is, the effective voltage applied to the liquid crystal layer becomes different depending on the position of a display area. This causes a so-called phenomenon “sticking”, and after the same image is displayed for a long time, if another image is displayed, the previous image remains as a residual image.